A 20th-century tale recounted the story of a woman who awoke one night believing she had discovered the meaning of life. When she looked at what she had scribbled the next morning, it was the four lines of doggerel you see above. But perhaps the dream was more profound than she thought. Perhaps it can help us understand the cultural reformation around sexual harassment that is exploding around us.

Let’s begin not with dreams but by going to the zoo. Penguins are monogamous. Females lay a large egg and then return to the sea to feed. The male incubates the egg for many weeks. When he is nearly dead from hunger and cold, the egg hatches and his mate returns. She regurgitates food to feed the fledgling while he goes to sea to feed. Many bird species are monogamous because the two sexes can share in bringing up the next generation.

Most of the 4,000 species of mammals are promiscuous or polygamous. Carrying the fetus in the womb and breastfeeding the newborn is a huge burden. As a result of this asymmetrical investment in reproduction, most male mammals fight one another for access to females instead of helping females reproduce. Think of bull elephant seals on the California coast battling for the opportunity to inseminate a much smaller female. The antlers we see on Christmas card renderings of reindeer are not to amuse Santa but to battle other male reindeer.

Now look at ourselves. Without doubt, an observant zoologist from Mars who had studied other mammals would conclude that human beings are promiscuous or polygamous. Why? Monogamous animals, like penguins, are usually the same size. Even zookeepers cannot tell males and females until one lays an egg. Male polygamous animals are nearly always larger than the females of the same species. The enormous silverback male gorilla, weighing between 300 and 400 pounds, is much stronger than the females, which are half his weight. Men are about 15 percent bigger than women and have greater upper body strength. Reproductive success for polygamous and promiscuous males depends on their ability to fight off rivals.

Male chimpanzees are also larger than females. Chimps are promiscuous, sometimes mating several times in a day with any ovulating female. We had a common ancestor with chimpanzees about 7 million years ago, and we can be sure that that ancestor had multiple sexual partners. I taught the evolution of human sexuality at UC Berkeley for many years, and I always described human beings as “polygamous animals struggling to be monogamous.”

As we evolved away from chimps, we began to walk upright and excelled at long-distance running. To do so, we needed a strong, bony pelvis. But we also evolved a baby with an extremely large brain. Pushing a baby with a big head through a narrow bony passage became increasingly painful and dangerous. Even so, the human baby is relatively immature at birth, unable to even hold up its head straight. In addition, to grow the brain after birth required more than year of breastfeeding with nutrient-rich milk.

I’ve been lucky enough to watch chimpanzees in Gombe Stream National Park, Tanzania, where Jane Goodall worked. Male chimps defend a territory, but they contribute nothing whatsoever to the upbringing of their offspring. When you watch the females, you soon appreciate that they are close to the limit of what it takes for one female to care for her infant alone. At some point in our own evolution, the burden of bringing up a helpless infant crossed a threshold, where a newborn’s survival was almost impossible without help from a partner.

Patterns of mating evolved. Not only was sexual intercourse used to reproduce, but it also began to express a new behavior — sexual love. All other mammals have no interest in sex unless the female is ovulating. Ovulation is commonly advertised in some way, as in female chimps where a huge swelling of the vulva indicates ovulation. In our species, ovulation is concealed — neither the man nor the woman knows when ovulation occurs. We are also unique in having sex on any day of the menstrual cycle. Given concealed ovulation, a Stone Age male could still have a one-night stand, but he would be unlikely to father a child — and, if he did, the infant might die without the additional care a woman needed and a man could offer. Not the best way to transmit your genes to the next generation — the ultimate metric of Darwinian evolution.

The alternative strategy was to have frequent intercourse with the same woman. Sooner or later the man would father a child. Now, bonded by sexual love, he would provide the extra nurturing the woman and infant needed to survive. Half of all American marriages end in divorce but, on average, they last long enough to conceive and secure the survival of at least one child.

The current recognition and condemnation of sexual harassment is one important step in the long journey from male-driven polygamy to female-driven monogamy. It is a revolution that will make the world a better place for men as well as for women. And most certainly for children.

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The Bixby Center for Population, Health, and Sustainability is dedicated to helping achieve slower population growth within a human right framework by addressing the unmet need for family planning. Learn more